The plant WAS closed in 2008, for all intents and purposes. Here is the timeline, if you would ever bother reading it (I doubt it):

June 3, 2008 – GM decides to close the Janesville plant, announcing that production will end by 2010, after months of rumors it might close. The press release declares, “Janesville, Wisconsin, will cease production of medium-duty trucks by the end of 2009, and of the Tahoe, Suburban and Yukon in 2010, or sooner.” Senators Herb Kohl and Russ Feingold, both Democrats, and Paul Ryan, whose House district includes Janesville, write the company urging it to reverse the decision.
September 2008 – Paul Ryan flies to Detroit to urge GM to reconsider its decision to close the plant. According to the Los Angeles Times, he pitched “a $224-million proposal that included roughly $50 million in state enterprise zone tax credits, local government grants worth $22 million and major contract concessions from the United Auto Workers union local.” Throughout, Ryan frequently speaks with GM chief Rick Wagoner.
Oct. 11, 2008 – Barack Obama comments on the Janesville closing. He does not promise to prevent the closing-in-progress, but instead declares he will “retool plants like the GM facility in Janesville” (emphasis mine) as president. Regardless of one’s views of the auto bailout, it has saved facilities like the Janesville one, if not the Janesville one in particular.
November/December 2008 – Congress weighs a bailout of GM and other automakers. One proposal, backed by Ryan and 31 other House Republicans, but not Mitt Romney, would have provided $15 billion in bridge loans. The bill passed the house but was not picked up by the Senate. The Bush administration declines to use TARP funds to rescue automakers, but approves a bridge loan on Dec.19, too late to save the Janesville plant.
Dec. 23, 2008 – Lacking a bailout, the plant closes. The plant holds a “final goodbye ceremony” as it builds its last SUV. In a statement to MSNBC, GM confirmed that the plant “was idled” in December. But — and this is where it gets confusing — winding down a plant takes time.
April 21, 2009 – The Janesville plant builds its last medium-duty truck and shuts down its last assembly line, completing the shutdown process started in June 2008.
In short, the Janesville shutdown commenced in June 2008. Once it was clear that aid wasn’t forthcoming in November, actual assembly lines were being shut down by December. It is true that Paul Ryan tried to get the Obama administration to save another plant, in Kenosha, which the Obama administration failed to do. Attacking Obama for that is fair. But hitting him for Janesville is dishonest. The first assembly line stopped rolling in December 2008. Workers unfurled banners declaring the “Last Vehicles Off the Janesville Line” at a “final goodbye ceremony,” The plant was closing regardless of what Obama did.
This is a very strange dispute, in a way. Mitt Romney wrote an op-ed in the New York Times under the title “Let Detroit Go Bankrupt,” and now his campaign is trying hard to fault Obama for not bailing out automakers aggressively enough. Not only that, but after the campaign’s repeated denunciation of the Obama administration for “picking winners,” Ryan is faulting Obama for not “picking a winner” not just among companies, but among plants. He’s attacking Obama for not using the government to micromanage GM’s affairs.